The present invention relates to the field of identification systems, and, more particularly, to a method for identifying electronic cards in an identification system.
Referring to FIG. 1, identification of electronic cards can be applied to any system having a fixed interrogation unit 10 capable of communicating or exchanging data messages with a plurality of mobile electronic cards 11-16. This communication is contactless, i.e., the messages are sent by radio frequency (RF). The transmission channel is thus formed by the ambient atmosphere.
An electronic card 11 may be an electronic module, a badge or a chip card. The electronic card 11 may be carried by an individual, a vehicle, an instrument, cattle, etc. The interrogation unit 10 may be contained in a base station, an access control terminal, an on-the-fly toll-gate, etc. As an alternative, the interrogation unit 10 is mobile and the electronic cards 11-15 are fixed. As another alternative, the interrogation unit 10 and the electronic cards 11-15 are both mobile. In all three cases, the electronic cards 11-15 may be located within a zone 20 centered around the interrogation unit 10. The zone 20 is the range in which messages may be exchanged. An electronic card 16 not within the zone 20 is outside the communications range of the fixed interrogation unit 10.
In FIG. 1, the boundary of this zone 20 is symbolized by a dashed line 25. The boundary of the zone 20 may vary in time, for example, due to the presence of foreign objects blocking propagation of the electromagnetic waves. However, it will be assumed that the boundary of the zone 20 is stable on a time scale when implementing an identification method. The volume of air contained in the zone 20 forms the channel for the transmission of messages exchanged between the interrogation unit 10 and the electronic cards 11-15. This channel is unique and has to be time-shared according to a communications protocol between the interrogation unit 10 and the electronic cards 11-15. This protocol is a master/slave type protocol. Each exchange of a message between the interrogation unit 10 (the master) and a specified card 11 (the slave) is initiated by the interrogation unit 10.
Implementation of such a system requires a method by which the interrogation unit 10 can identify the electronic cards 11-15 present in the zone 20. Hereinafter, this zone 20 is called the investigation zone because it represents the area in which the electronic cards 11-15 must be detected and identified. To identify an electronic card is the same as identifying its identification number.
For each electronic card 11, an identification number is assigned. In other words, no two electronic cards have the same identification number. A control message sent by the interrogation unit 10 for a specified electronic card contains the identification number for enabling definition of the electronic card for which the control message is intended. Due to mobility of the electronic cards 11-15 and/or of the interrogation unit 10, there is no a priori knowledge of whether the electronic cards 11-15 are in the zone 20. As the case may be, the interrogation unit 10 does not know how many and which electronic cards are present.
In the prior art, identification methods of this kind have already been proposed. These known methods implement a tree-like iterative algorithm used for a bit-by-bit reconstruction of the identification number of each card present in the investigation zone. More specifically, these methods include an exchange of interrogation messages sent by the management unit to a group of electronic cards 11-15 defined by the set of cards whose identification number includes an already identified common portion. Each concerned electronic card present in the investigation zone 20 responds by sending response messages, including its complete identification number. The interpretation of the response messages received by the interrogation unit 10 at each iteration of the algorithm makes it possible to move forward in the tree of the identification numbers by identifying the value of an additional bit of the identification number of at least one of the electronic cards. At the next iteration, the group of cards for which the interrogation message is sent by the interrogation unit 10 is smaller because the already identified common radical now includes an additional bit. These known methods suffer from two drawbacks.
The first drawback results from the length of the messages exchanged, especially the response message sent by the electronic cards. In certain applications where the number of electronic cards is large, the identification numbers may be encoded on 16 or even 32 bits. This means that a large amount of time is needed to identify all the cards present in the investigation zone.
The second drawback lies in the fact that, at each iteration, several electronic cards can simultaneously emit an answering signal if their respective identification numbers have a common portion. This results in a collision because the electromagnetic signals simultaneously transmitted on the transmission channel corrupt one another. There presently exist known methods that make it possible to overcome this collision problem. There are known methods that even exploit this phenomenon according to a particular identification algorithm. These methods may provide satisfactory results. However, they can be used only for certain types of encoding of binary information. In particular, these known methods can be applied only to systems that use a type of encoding with a return to zero of the signal in the length of the bit. This is the so-called xe2x80x9cRZxe2x80x9d encoding, such as used in the Manchester type encoding scheme.
An object of the present invention is to provide a method for the identification of electronic cards that is fast and efficient, and can be applied to systems using various types of encoding of the binary information.
The invention provides a method for the identification of electronic cards where each card is assigned an identification number encoded on M bits distributed into P blocks of Q bits. According to the invention, this method includes a block-by-block reconstruction of the identification numbers of the cards present in an investigation zone according to an arborescent iterative algorithm. At each iteration, the reconstruction comprises the following steps.
An interrogation unit transmits an interrogation message for certain electronic cards authorized to respond. All the cards are authorized to respond at the first iteration, and the only cards authorized to respond at each subsequent iteration are the unidentified cards that have an identification number of which at least one block of bits has been identified in a prior iteration. Each card present and authorized to respond then transmits a response message intended for the interrogation unit. The response message includes at least one service bit sent in a narrow time window whose positioning in a sequence of 2Q successive identical windows indicates the value of an as yet unidentified block of bits of its identification number.
According to one advantage of the invention, at each iteration the response signal sent by the electronic cards that respond includes only one or more service bits in a reduced number. Namely, a number significantly smaller than the number M of bits of the identification number sent according to the prior art methods. The duration of transmission of the response message is therefore relatively short. Consequently, the identification of the cards present in the investigation zone is quicker. The expression xe2x80x9cservice bitxe2x80x9d refers to a bit whose logic value bears no useful information per se. The service bit or bits have an identical logic value corresponding to the electronic card that responds.
Furthermore, the transmission of the same response message simultaneously by several electronic cards gives rise to a collision which has no effect on executing the identification algorithm. This characteristic, as well as other characteristics and advantages of the invention, shall appear more clearly from the following detailed description. This description is given purely by way of an illustration and must be read with reference to the appended drawings.